The present invention relates to powder metallury (P/M) stainless steel powders and compacts therefrom, and more particularly to improving the corrosion resistance of such powders and compacts.
Heretofore, poor corrosion resistance of such compacts has been attributed mainly to the porosity found within the compacts, thus most techniques for overcoming corrosion problems have been aimed at closing the porosity. Prior techniques aimed at minimizing the surface porosity of the compacts made from such stainless steel powders include mechanical closure treatment, plastic impregnation, surface coatings, or passivation treatments. Each of these techniques has some limitation as to its effectiveness in addition to raising the cost of the final product. Other proposals aimed at improving the corrosion resistance of stainless steel powder compacts concentrate on compacting and sintering parameters. These proposals generally state that the sintering conditions and sintering atmosphere have a marked influence on the corrosion properties of the powder compact; however, most of the experimental results reported in these proposals are inconsistent. For example, Kalish and Mazza ("An Evaluation of Dissociated Ammonia and Hydrogen Atmospheres for Sintering Stainless Steel", Journal of Metals, TRANS, AIME., February 1955, pages 304-310) state that sintering in hydrogen provides a more corrosion-resistant compact than sintering in dissociated ammonia which gives rise to a fourfold increase in the corrosion rate of the compact. Stosuy et al (Metal Progress, Vol. 91, pages 81-85, 1967) and Jones ("The Effect of Processing Variables on the Properties of Type 316L Powder Compacts", Progress in Powder Metallurgy, Vol. 30, pages 25-50, April 1974) report that an optimum combination of mechanical properties and corrosion resistance of the compact can be obtained by sintering the compact in dissociated ammonia. Furthermore, Sands et al ("The Corrosion Resistance of Sintered Austenitic Stainless Steel", Modern Developments in Powder Metallurgy, Vol. 2., H. H. Hausner, ed., Plenum Press, New York, N.Y., pages 73-83, 1966) report that while sintering in vacuo always gives a good corrosion-resistant product, sintering in either dissociated ammonia or hydrogen can lead to loss of corrosion resistance. With respect to sintering in dissociated ammonia, there is some evidence which points to the probable formation of chromium nitrides during cooling of the resultant dissociated ammonia-sintered compact which results in localized chromium depletion and, thus, loss of effective corrosion protection of the sintered compact. Certainly, the inconsistencies among these various citations demonstrate the confusion prevalent in the art with regard to the corrosion resistance of stainless steel powder and compacts made therefrom.
Another proposal for improving the corrosion resistance of stainless steel powder compacts is disclosed in Japanese Tokkai 35708 (1977). According to said disclosure, the addition of a small proportion of tin, and optionally copper also, improves the corrosion resistance of stainless steel compacts. This reference, however, does not provide any basis for concluding that other additions might also influence corrosion resistance and may even have negative implications.
It now has been discovered that stainless steel powders atomized in an oxidizing environment (e.g. a conventional water atomization process) are surface-enriched in silicon oxides (primarily silicon dioxide) and, thus, surface-depleted in chromium. Such depletion or loss of chromium about or at the surface of the powder is believed to lead to the poor corrosion resistance of the powder and more importantly of the ultimate compact made from such powder. "Surface-enrichment" in this application means that the composition of the powder or compact about its surface is substantially different from the bulk composition of the powder with a marked increase of silicon oxides being located about the surface of the powder (or compact) or within close proximity to the surface (e.g. about 0.5 micrometers into the powder or compact from the surface). Effective removal of such silicon oxides (the term "silicon oxide" is intended to refer to the various forms of oxidized silicon which primarily is believed to be silicon dioxide, but is not intended to be a limitation of the present invention) about the surface provides unexpected superior corrosion resistance of the powder and compact made therefrom.